1. Field of the Invention
This invention relates generally to a system and method for preventing water build-up in the cathode flow channels of a fuel cell stack using cell voltage pattern recognition and, more particularly, to a system and method for periodically providing pulsed cathode air flow to the cathode side of a fuel cell stack at low stack current power density to determine the pattern of the cell voltages during the pulses to prevent water build-up in the cathode flow channels.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer-electrolyte proton-conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs require certain conditions for effective operation, including proper water management and humidification.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For the automotive fuel cell stack mentioned above, the stack may include about two hundred or more fuel cells. The fuel cell stack receives a cathode reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
As is well understood in the art, the membranes within a fuel cell need to have a certain relative humidity so that the ionic resistance across the membrane is low enough to effectively conduct protons. During operation of the fuel cell, moisture from the MEAs and external humidification may enter the anode and cathode flow channels. At low cell power demands, typically below 0.2 A/cm2, water accumulates within the flow channels because the flow rate of the reactant gas is too low to force the water out of the channels. As the water accumulates, it forms droplets that continue to expand because of the relatively hydrophobic nature of the plate material. The contact angle of the water droplets is generally about 80°-90° in that the droplets form in the flow channels substantially perpendicular to the flow of the reactant gas. As the size of the droplets increases, the flow channel is closed off, and the reactant gas is diverted to other flow channels because the channels are in parallel between common inlet and outlet manifolds.
Because the reactant gas may not flow through a channel that is blocked with water, the reactant gas cannot force the water out of the channel. Those areas of the membrane that do not receive reactant gas as a result of the channel being blocked will not generate electricity, thus resulting in a non-homogenous current distribution and reducing the overall efficiency of the fuel cell. As more and more flow channels are blocked by water, the electricity produced by the fuel cell decreases, where a cell voltage potential less than 200 mV is considered a cell failure. Because the fuel cells are electrically coupled in series, if one of the fuel cells stops performing, the entire fuel cell stack may stop performing.
Low performing cells, especially at low stack power output, is a problem in fuel cell applications. Low performing cells typically produce more water than other cells, and can lead to flow channel flooding. One flooded cell can start a downward spiral of operation that may ultimately lead to stack failure, especially during low-power operation. As discussed above, the most common cause of low performing cells and fuel cell stack failure is significant cell-to-cell variation as a result of water holdup caused by stochastic variations in gas behavior dynamics.
It is usually possible to purge the accumulated water in the flow channels by periodically forcing or pulsing the reactant gas through the flow channels at a higher flow rate than is necessary to provide the desired output power. For example, it is known in the art to pulse the cathode air through the flow channels when the fuel cell stack has been continuously operating at low power demands, such as would occur when the vehicle is idling for an extended period of time. For example, the cathode air may be pulsed to a level for half stack power every 3-5 minutes. By pulsing the cathode air in this manner, water is forced out of the channels. However, the increased airflow dries the membranes causing problems with expansion and shrinkage of the membrane. Also, an increased airflow increases the parasitic power applied to the air compressor, thereby reducing overall system efficiency. Further, this technique often comes too late to prevent the cell from failing.
The division of gases into a large number of cells is seldom equal. The cells that receive less gas will see a higher relative humidity and the cells with more gas will have a lower relative humidity. If a cell's relative humidity increases above 100%, water will collect in the cathode flow field channels, further decreasing air flow and compounding the problem until the cell fails or the water is removed.